1. Field of the Invention
The present invention is directed to low flow fuel nozzles for use in burners, such as oil burners, and more particularly, to an air assisted simplex fuel nozzle that is adapted for fuel modulation and uses auxiliary assist air to atomize the fuel.
2. Background of the Related Art
Conventional burners used in home heating applications generally include a fuel supply conduit connected at one end to a fuel supply pump and terminating at the other end at a fuel nozzle where the fuel is dispensed as an oil spray. The spray nozzle functions to mix the fuel with air that has been delivered by a motor powered blower. A burner-mounted ignition system is connected to an ignition apparatus that is located adjacent to the fuel nozzle near the exit where it ignites the atomized fuel-air mixture.
Typically, home heating applications require low flow rates (approximately 0.5 gph to 1.0 gph) of finely atomized fuel. Moreover, extremely low fuel flow rates (less than 0.5 gph) are desirable in applications where the volume of air to be heated is small, such as in a trailer home or small office.
Several known techniques exist for atomizing fuel. One conventional method of atomizing fuel is “pressure atomization,” whereby high velocity fuel is injected into relatively low velocity air. The interaction between the fuel and air shreds the fuel into fine droplets and subsequently greatly increases the fuel's surface area. The fine droplets and large surface area-to-volume ratio enhance chemical reaction rates that are beneficial to many processes. The disadvantage of using pressure atomization for low fuel flow rates is that the fluid passage size has to be very small to generate the hydraulic pressure required for atomization. Small fluid passage sizes are difficult to manufacture and are detrimental to product life due to a propensity to plug the fuel passage with contamination. When the passage size is maintained at some minimal value that is deemed acceptable for contamination resistance, the resultant hydraulic pressure associated with such a reduced fueling rate is so low that atomization is poor or nonexistent and fuel distribution is substandard.
An alternative method for atomizing fuel is to inject low velocity fuel into a relatively high velocity air stream. This method is generally referred to as “air blast atomization”. This method overcomes the minimum fluid passage size and low fuel pressure issues associated with “pressure atomization” as long as there is sufficient kinetic energy in the atomizing air stream to properly break up the fuel. In certain applications, the air stream does not have sufficient energy for atomization or there are operating modes where the air stream has limited energy for atomizing the fuel. When the atomizing air energy is low or insufficient, the result is the same as that of the low flow pressure atomizer; poor or nonexistent atomization and poor fuel distribution.
For applications where the required fuel flow rate is too low for effective pressure atomization and where there are no air blast atomizing air streams with sufficient energy across the application's entire operating range; an air assist system can be used. Air assist atomizers typically utilize a relatively high-pressure, high velocity air from an external source to augment the atomization process. Because the air assist atomizer uses an external source (e.g., a compressor), it is important to keep the air flow rate to a minimum in order to minimize the cost of the auxiliary air system. Thus, air assist atomizers are characterized by their use of a relatively small quantity of very high velocity air. The use of kinetic energy from the auxiliary air circuit to break up the fuel droplets provides very good atomization and fuel distribution at very low fuel flow rates. The low fuel pressure and fuel velocities associated with low fuel flow rates are not detrimental in an air assisted atomizer; in fact, a low fuel exit velocity as compared to a high air assist velocity provides the greatest relative velocity between the two fluids and promotes good atomization.
A siphon nozzle, shown in FIGS. 1a and 1b, is an example of a known method for using assist air to atomize the fuel. The siphon nozzle routes air from an external source and directs it towards a fuel delivery feature which is normally a simple orifice. The air circuit is configured to create a low pressure region at the fuel delivery outlet which draws the fuel into the air steam. The amount of fuel drawn into the air is related to the lift height of the fuel above a fuel reservoir and the amount of air moving through the nozzle. While siphoning is a very effective method of atomizing fuel it has a limited range of fuel modulation. In a siphon nozzle, if the fuel supply is pressurized to increase the fuel flow rate then the simple orifice creates a plain jet of fuel which inhibits fuel atomization. Also when a simple orifice is used that does not impart a swirl or spin to the fuel, the resultant spray pattern tends to be a solid cone which may or may not be a match for a particular application.
Another example of a prior art device that uses assist air to atomize the fuel is an “Airo” nozzle, shown in FIGS. 2a and 2b. This concept uses internal mixing of pressurized fuel and air to atomize the fuel. With internal mix atomizers there can be interactions between the fuel circuit and air circuit. For instance, a change in the fuel flow rate may have an effect on the air flow rate or an increase in air pressure may change the fuel spray angle. The “Airo” concept will atomize very low flow rates of fuel, but because of the interactions between the fuel and air circuits, may require more complex controls to properly modulate the fuel and air circuits.
Therefore, there is a need for a low flow fuel nozzle for use in burners, such as oil burners that is easily modulated and uses assist air to atomize the fuel.